Luminaires with automated and remotely controllable functionality are well known in the entertainment and architectural lighting markets. Such products are commonly used in theatres, television studios, concerts, theme parks, night clubs, and other venues. A typical product will commonly provide control over the pan and tilt functions of the luminaire allowing the operator to control the direction the luminaire is pointing and thus the position of the light beam on the stage or in the studio. Typically, this position control is done via control of the luminaire's position in two orthogonal rotational axes usually referred to as pan and tilt. Many products provide control over other parameters such as the intensity, color, focus, beam size, beam shape, and beam pattern. FIG. 1 illustrates a typical multiparameter automated luminaire system 10. These systems typically include a plurality of multiparameter automated luminaires 12 which typically each contain on-board a light source (not shown), light modulation devices, electric motors coupled to mechanical drive systems, and control electronics (not shown). In addition to being connected to mains power either directly or through a power distribution system (not shown), each automated luminaire 12 is connected in series or in parallel to data link 14 to one or more control desks 15. An operator typically controls the automated luminaire system 10 through the control desk 15.
To achieve the high brightness needed for such systems it is common to utilize High Intensity Discharge lamps (HID). Short and medium arc HID lamps produce light from a plasma cloud produced by an electrical arc that is maintained between two adjacent electrodes within a sealed quartz envelope. FIG. 2 shows an example of an HID lamp 30 that may be used. Electrodes 32 and 33 are enclosed within sealed quartz envelope 38 and connected by lead wire 35 to a base 36 and electrical connections 37. The HID lamps used in entertainment lighting luminaires often use very small arc gaps 34 between the two electrodes, of the order of 1 to 5 mm, to provide a low etendue light source that facilitates the design of a high quality optical system for projecting images and colors. The spectrum of light emitted by the lamp is produced by the ionization and emission of a mix of rare earths and gases that are contained within the envelope 38. The emission spectra of each of these components, when heated to the plasma temperatures of the arc, combine to produce an overall emission spectrum for the lamp. The lamp manufacturer carefully selects the mix of constituents for the lamp fill in order to produce a white light output with a spectrum that approximates to that of a black body emitter at the desired color temperature. For example, it is common to manufacture HID lamps with a target color temperature of 5600 Kelvin (K), or daylight. It is also common to produce lamps with target color temperatures of 3200 K, 7000 K, 10000 K, and other white points as commonly used in the entertainment lighting business for television cameras, film cameras, or a live audience.
A significant problem with such lamps is maintaining the stability of the desired target color temperature. Small changes in the arc gap, as the electrodes burn away, and fluctuations in the temperature of the lamp envelope can make significant changes to the precise mix of constituents that are emitting spectra to the combined spectrum. For example, as the temperature drops within the envelope then some constituents that emit specific wavelengths of light may drop out of the ionization cloud, or alter their output, thus affecting the resultant output spectrum and thus the output color temperature. Lamp manufacturers may attempt to mitigate this variability by enclosing the inner quartz envelope 38 within a second outer envelope (not shown) to provide rudimentary temperature control. However, such designs are still not stable and the color temperature may vary significantly.
It is also common to desire to change the power consumed by the lamp, in order to control its brightness. Unfortunately any change in lamp power also affects the operating temperature of the lamp that, in turn, will affect the output color temperature. Prior art systems have utilized fan cooling systems to attempt to stabilize the lamp temperature, but these have been ineffective and slow to operate, allowing large changes in the lamp output color temperature that were visible to the audience.
It would be advantageous to provide a system that was capable of providing continuous and dynamic control of the temperature of the envelope of an HID lamp in order to stabilize the output color temperature of the lamp.